A map of how boundaries of the states and territories of the United States changed over time. This map is limited to the contiguous United States, and so does not include Alaska (purchased from Russia in 1867, organized as a territory in 1912, recognized as a state in 1959) or Hawaii (annexed 1898, organized as a territory in 1900, recognized as a state in 1959), or the many imperial ventures of the United States in Cuba, the Philippines, and other places. For reasons of scale, the District of Columbia is not shown, but DC was created in 1791, ceded territory back to Virginia in 1846, and was organized as a territorial government from June 1, 1871 to June 20, 1874 (see chronologies from the and Atlas of Historical County Boundaries and the National Archives). The start and end dates show most of the major boundary changes for the first 48 states; there have been some minor boundary changes since 1912. The dates for Reconstruction start with the First Reconstruction Act and end with the date each state was re-admitted to Congress. This map is intended primarily as a base layer for other historical maps that show change over time (an example). Keep in mind that ‘the map is not the territory’, and that the continent was not empty of other peoples or polities.
The underlying data about the political boundaries is from John H. Long, et al., Atlas of Historical County Boundaries, Dr. William M. Scholl Center for American History and Culture, The Newberry Library, Chicago (2010), though I have added dates for secession and Reconstruction. The data for the North American continent is from Natural Earth.
Map by Lincoln Mullen. Code licensed MIT. Suggestions welcome here.